Method for producing radiation sensitive alkali halide crystals



United States atent The invention described herein may be made and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a method for producing alkali halide crystals that undergo optical changes as a result of irradiation with X-rays, gamma rays, electrons and by UV light.

Alkali halides have not demonstrated heretofore permanent optical effects upon exposure to ionizing radiation, and therefore, they have not been particularly suitable for use in dosimetry. When potassium hydride, for instance, is introduced in a crystal of potassium bromide, the crystal becomes sensitized and upon irradiation by X- ray gamma rays, etc., a blue color appears, which is proportional to the irradiation dose, but the color fades rapidly, even at room temperature, and it is completely removed at about 100 C.

It is now well known that F-centers, which are electrons trapped at negative ion vacancies, are responsible for the color produced in crystals. An alkali halide crystal which is baked at a high temperature in an excess atmosphere of alkali metal vapor and then cooled rapidly to room temperature becomes deeply colored. in the crystals produced in accordance with the present invention, F-centers are initially suppressed and no appreciable coloration appears therein; F-centers are developed in the crystals only as a result of radiation, and the color produced thereby is indicative of the amount of radiation which the crystal has received.

Accordingly, it is an object of the present invention to provide a method for producing alkali halide crystals with new and useful optical effects.

Another object of the present invention is to produce substantially clear, colorless alkali halide crystals which undergo coloration following irradiation, and heating of said crystal.

A further object of the present invention is to provide a method for sensitizing doped alkali halides to produce a color therein by means of radiation and heating.

A still further object is to, provide a novel technique for producing crystals ofalkali halides in which F-centers are initially suppressed but which are formed by means of radiation, and the colloidal color produced after heating is relative to the amount of radiation received.

And a still further object of the present invention concerns a method for producing crystals which are susceptible to the growth of colloidal particles by ionizing radiation.

In conjunction with the foregoing objects, the method described herein provides a crystal structure which is readily adaptable for use in detecting ionizing radiation. In fulfilling this purpose, a dosimeter of considerable sensitivity and efiectiveness is described in a copending application Serial No. 803,040, filed March 30, 1959, said dosimeter device utilizes the novel crystals produced in accordance with the teachings of. the present invention.

The nature of the invention and further aspects. of novelty and features thereof will. be better appreciated from the following detailed disclosure.

The method comprises a technique for growing ingots of alkali halides, substantially colorless, clear crystals of ice.

NaCl, KCI, KBr, etc., containing small quantities of hydroxyl ions and for treating said crystals further in the vapor of an alkali metal to produce thereby novel crystals that are sensitized to incident ionizing radiation. Under the influence of a radiation and with thermal stimulation at a temperature in the neighborhood of about 360 C. or more, said crystals undergo a coloration. Specifically, the crystals produced in accordance with the present invention become colored under the stimuli of radiation and heat. The crystal material may be heated either concurrently with or subsequent to irradiation. The color produced in the crystals is stable: It will not fade with time, in the light, nor over a wide temperature range. Moreover, the color imparted to said crystals is related to the dose of radiation received, a characteristic which will make the present crystal composition useful as a detecting element in dosimetry.

Single crystals of NaCl, KBr, KCl, etc., may be grown by any of the methods employed in crystal-growing technology. It has been found convenient to grow the alkali halides by the Kyropoulos process in which a single crystal rod isproduced by lifting the growing crystal out of the melt. Crystals grown in this manner are found to possess a necessary constituent in the crystal lattice which virtually controls the appearance of color and provides the means for sensitizing the crystal composition toward radiation. A specific requirement, therefore, during the crystal growth, and one distinctly in contrast with the precaution normally observed in crystal technology is the provision for growing alkali halide crystals in the presence of moist air. Crystals grown in air contain intheir crystal lattice a certain amount of OH ions due-to the water content present in air. The small hydroxyl ion content of air-grown crys als has been found to be effective for producing the desired results in accordance with the present invention. NaCl crystals have been grown successfully from a melt of pure sodium chloride in the presence of normal room atmosphere. Moist air may be especially controlled and circulated for this purpose to contact both the melt and the growing crystal. The average hydroXyl ion content of air-grown crystals has been found to be about 30 ppm. based on the halide ion. A reasonable concentration of OH ion which may be. incorporated in this manner is about 10 to hydroxyl ions per million halide ions in the crystal. The OH ion concentration withinthis rangezis sufficient to produce the desired effects described below.

More conveniently, the hydroxyl ion is introduced in the melt of an alkali halide from a predetermined amount of an alkali hydroxide. For example,NaOH is added in about 0.1. mole, percent directly to a NaCl melt. methodis found preferable. for incorporating OH into a growing crystal. since any specified ionconcentration may be introduced thereby; larger. OH- concentrations which may be-introduced directly into the melt afford a better, opportunity for the growing crystal to develop with greater concentrations of. OH. ions in the crystal lattice. In a KCl melt, KOH is added therein in 0.14 mole percent; the OH- concentration which. is successfully incorporated: in the KOH crystalis calculated at about 30 parts per million. Other 'melt. methods, e.g. Stockbarger, Bridgman, etc., may also be used, to introduce the hydroxyl ioninto the crystal during the crystal growth. The hydroxyl ion may also bejntroduced into pure alkali halide crystals by diffusion, i.e., baking the crystal in moist atmosphere.

After the hydroxyl has been incorporated. in the crystal structure, the crystal is then treated by a method which issimilar to an additive coloration technique which heretofore has been used'as a means for. introducing F-centers in crystals and is referred to by R. W. Pohlin Proceedings of the P-hysical-Society 49 pp. 3-31 (1937). The tech tributed to the growth of colloidal particles.

nique as originally developed induced deep colors directly in alkali halide crystals. In the method of the present invention the crystal is baked at high temperature and cooled slowly to prevent formation of F-centers. The crystal, therefore remains substantially colorless; the F- centers are suppressed during the baking and cooling steps, but they are formed upon exposure of the crystal to ionizing radiation.

. In carrying out the present treatment, the crystal grown with OH in its structure is placed in a sealed container with an excess amount of cation metal of the alkali halide, or any other suitable alkali metal. The metal present in the sealed container is added in excess of the amount required to maintain vapor pressure equilibrium at the baking temperature. The crystal is baked in the presence of metal vapor at a temperature sutficient to allow the difiusion, of alkali metal atoms into the crystal lattice. The duration of baking the crystal depends on (1) the dimensions of the crystal, (2) the particular alkali metal present in the container, and (3) the temperature at which the baking operation is maintained. A sodium chloride crystal of about 1 cc. in size is baked in excess sodium vapor for about 18 hours. When the baking temperature is increased to 750 C., the baking operation can be completed in about 2 hours.

Care should be exercised following the baking operation to cool the crystal slowly in order to prevent the formation of F-centers. In the prior art crystals treated in metal vapor at high temperatures were quenched in order to produce F-centers and the accompanying deep coloration. Rapid cooling causes the dissociation products'of the OH- to supply the electrons and vacancies that form F-centers. In the present invention it is desirable to cool slowly to prevent formation of F-centers and to minimize color formation as much as possible. The ingot of 1 cc. in size requires several hours of cooling to provide a crystal which is substantially colorless and free from F-centers in its structure.

Alkali halide crystals produced in the manner described herein contain a stoichiometric excess of alkali metal and the dissociation products of the hydroxyl ion. One of the constituents in the lattice is the hydride ion (H). The novel crystal is now sensitized to ionizing radiation and is capable of forming F-centers when radiation of about 6 ev. or more penetrates the crystal. Under the effect of X-rays, gamma rays, ultraviolet light or electrons the crystal acquires F-centers relative .to the radiation dose, and upon heating to about 300 C. or more, a color is thereby produced in the crystal. Coloration is at- Under the influenceof heat F-centers promote the growth of a col- Iloidal dispersion of alkali metal. The color so produced does not fade, nor is, it altered by cxposureto sunlight and other types of radiation.

Hence it will be apparent that the optical properties of alkali halides are aflected by the hydroxyl ion which is incorporated therein. The color produced by irradiation is related to the hydroxyl ioncontent which was originally present in the crystal. -The' sensitivity of the crystal to ionizing radiation depends, moreover, on the hydroxyl ion. concentration which was incorporated therein during crystal growth.

Obviously manvmodifications and variations of the 7 present invention are possible in the light of the above teachings. It is therefore understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is: 7

1. Themethod of preparing a susbtantially colorless alkali halide crystal sensitive .to ionizing radiation comprisinggrowing a single crystal from a melt of alkali halide in the presence of moist air to incorporate from about 10 to 100 hydroxyl ions per million of negative ions in the crystal baking said crystal in excess alkali m t lrv p r at a. temperature andfor a period of time sufficient to introduce alkali metal into the crystal and cooling said baked crystal slowly to room temperature. 2. The method of preparing a substantially colorless alkali halide crystal sensitive to ionizing radiation comprising growing a single crystal from a melt of alkali halide containing in said melt a predetermined amount of alkali hydroxide to incorporate hydroxyl ions into said crystal, baking said crystal in excess alkali metal vapor at a temperature and for a period of time sufiicient to introduce alkali metal into the crystal and cooling said baked crystal slowly to room temperature.

3. The method of preparing a substantially colorless sodium chloride crystal sensitive to ionizing radiation comprising growing a single crystal from a melt of sodium chloride containing in said melt a predetermined amount of alkali hydroxide to incorporate hydroxyl ions into said crystal, baking said crystal in excess alkali metal vapor at a temperature in the range of about 650 C. for a time sufficient to introduce alkali metal into the crystal and cooling said baked crystal slowly to room temperature.

4. The method of sensitizing a substantially colorless alkali halide crystal containing from about to 100 hydroxyl ions per million of negative ions in the crystal comprising baking said crystal in excess alkali metal vapor at a temperature and for a period of time sufiicient to introduce alkali metal into the crystal and cooling said baked crystal slowly to room temperature whereby said crystal is'capable of developing color therein on exposure to ionizing radiation and following heating to a temperature of about 300 C. or more.

5. The method of sensitizing a substantially colorless sodium chloride crystal containing from about 10 to 100 hydroxyl ions per million chloride ions in the crystal comprising baking said crystal in alkali metal vapor at a temperature in the range of 650 to 750 C. for a time suflicient to introduce alkali metal into the crystal and cooling said baked crystal slowly to room temperature whereby said crystal is capable of developing color therein on exposure to ionizing radiation and following heating to a temperature of about 300 C. or more.

6. The method of preparing a substantially colorless sodium chloride crystal sensitive to ionizing radiation comprising. growing a single crystal from a melt of sodium chloride containing in said melt about 0.1 mole percent of sodium hydroxide, baking the sodium chloride crystal in excess sodium vapor at a temperature in the range of about 650750 C. for a time sufiicientto introduce sodium atoms into the crystaland cooling the baked crystal slowly to room temperature.

7. The method of preparing a substantially colorless sodium chloride crystal sensitive to ionizing radiation comprising growing a'single crystal from a melt of sodium chloride in the presence of moist air to incorporate from about 10 to 100 hydroxyl ions per million chloride ions in the crystal, baking said crystal in excess alkali metal vapor at a temperature in the range of about 650- 750" C. for a time suflicient to introduce sodium atoms into the crystal and cooling the baked crystal slowly to room temperature.

References Cited in the file of this patent UNITED STATES PATENTS 2,673,934 Friedman Mar. 30, 1954 2,689,308 Land Sept. 14, 1954 2,763,786 Mauer et al Sept. 18, 1956 2,882,414 Joyner et al Apr. 14, 1959 2,882,415 Pressau Apr. 14, 1959 OTHER REFERENCES Fehlner: Growing Crystals, Jour. of Chem. Education, vol; 33, No. 9, September 1956, pp. 449-451.

Halperin et al.: Effect of Thermal Pretreatment on the Thermoluminescence of KCl Crystals, Physical Review, vol. 113, No. 3, February 1, 1959, pages 762-766. 

6. THE METHOD OF PREPARING A SUBSTANTIALLY COLORLESS SODIUM CHLORIDE CRYSTAL SENSITIVE TO IONIZING RADIATION COMPRISING GROWING A SINGLE CRYSTAL FROM A MELT OF SODIUM CHLORIDE CONTAINING IN SAID MELT OF 0.1 MOLE PERCENT OF SODIUM HYDROXIDE, BAKING THE SODIUM CHLORIDE CRYSTAL IN EXCESS SODIUM VAPOR AT A TEMPERATURE IN THE RANGE OF ABOUT 650-750*C. FOR A TIME SUFFICIENT TO INTRODUCE SODIUM ATOMS INTO THE CRYSTAL AND COOLING THE BAKED CRYSTAL SLOWLY TO ROOM TEMPERATURE. 